[v4,00/14] RAM_NORESERVE, MAP_NORESERVE and hostmem "reserve" property

Message

David Hildenbrand March 19, 2021, 10:12 a.m. UTC

Some fixes for shared anonymous memory, cleanups previously sent in other
context (resizeable allocations), followed by RAM_NORESERVE, implementing
it under POSIX using MAP_NORESERVE, and letting users configure it for
memory backens using the "reserve" property (default: true).

MAP_NORESERVE under Linux has in the context of QEMU an effect on
1) Private/shared anonymous memory
-> memory-backend-ram,id=mem0,size=10G
2) Private fd-based mappings
-> memory-backend-file,id=mem0,size=10G,mem-path=/dev/shm/0
-> memory-backend-memfd,id=mem0,size=10G
3) Private/shared hugetlb mappings
-> memory-backend-memfd,id=mem0,size=10G,hugetlb=on,hugetlbsize=2M

With MAP_NORESERVE/"reserve=off", we won't be reserving swap space (1/2) or
huge pages (3) for the whole memory region.

The target use case is virtio-mem, which dynamically exposes memory
inside a large, sparse memory area to the VM. MAP_NORESERVE tells the OS
"this mapping might be very sparse". This essentially allows
avoiding having to set "/proc/sys/vm/overcommit_memory == 1") when using
virtio-mem and also supporting hugetlbfs in the future.

virtio-mem currently only supports anonymous memory, in the future we want
to also support private memfd, shared file-based and shared hugetlbfs
mappings.

virtio-mem features I am currently working on that will make it all
play together with this work include:
1. Introducing a prealloc option for virtio-mem (e.g., using fallocate()
   when plugging blocks) to fail nicely when running out of
   backing storage like huge pages ("prealloc=on").
2. Handling virtio-mem requests via an iothread to not hold the BQL while
   populating/preallocating memory ("iothread=X").
3. Protecting unplugged memory e.g., using userfaultfd ("prot=uffd").
4. Dynamic reservation of swap space ("reserve=on")
5. Supporting resizable RAM block/memmory regions, such that we won't
   always expose a large, sparse memory region to the VM.
6. (resizeable allocations / optimized mmap handling when resizing RAM
    blocks)

v3 -> v4:
- Minor comment/description updates
- "softmmu/physmem: Fix ram_block_discard_range() to handle shared ..."
-- Extended description
- "util/mmap-alloc: Pass flags instead of separate bools to ..."
-- Move flags to include/qemu/osdep.h and rename to "QEMU_MAP_*"
- "memory: Introduce RAM_NORESERVE and wire it up in qemu_ram_mmap()"
-- Adjust to new flags. Handle errors in mmap_activate() for now.
- "util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE under Linux"
-- Restrict support to Linux only for now
- "qmp: Include "reserve" property of memory backends"
-- Added
- "hmp: Print "reserve" property of memory backends with ..."
-- Added

v2 -> v3:
- Renamed "softmmu/physmem: Drop "shared" parameter from ram_block_add()"
  to "softmmu/physmem: Mark shared anonymous memory RAM_SHARED" and
  adjusted the description
- Added "softmmu/physmem: Fix ram_block_discard_range() to handle shared
  anonymous memory"
- Added "softmmu/physmem: Fix qemu_ram_remap() to handle shared anonymous
  memory"
- Added "util/mmap-alloc: Pass flags instead of separate bools to
  qemu_ram_mmap()"
- "util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE"
-- Further tweak code comments
-- Handle shared anonymous memory

v1 -> v2:
- Rebased to upstream and phs_mem_alloc simplifications
-- Upsteam added the "map_offset" parameter to many RAM allocation
   interfaces.
- "softmmu/physmem: Drop "shared" parameter from ram_block_add()"
-- Use local variable "shared"
- "memory: introduce RAM_NORESERVE and wire it up in qemu_ram_mmap()"
-- Simplify due to phs_mem_alloc changes
- "util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE"
-- Add a whole bunch of comments.
-- Exclude shared anonymous memory that QEMU doesn't use
-- Special-case readonly mappings

Cc: Peter Xu <[e-mail]>
Cc: "Michael S. Tsirkin" <[e-mail]>
Cc: Eduardo Habkost <[e-mail]>
Cc: "Dr. David Alan Gilbert" <[e-mail]>
Cc: Richard Henderson <[e-mail]>
Cc: Paolo Bonzini <[e-mail]>
Cc: Igor Mammedov <[e-mail]>
Cc: "Philippe Mathieu-Daudé" <[e-mail]>
Cc: Stefan Hajnoczi <[e-mail]>
Cc: Murilo Opsfelder Araujo <[e-mail]>
Cc: Greg Kurz <[e-mail]>
Cc: Liam Merwick <[e-mail]>
Cc: Marcel Apfelbaum <[e-mail]>

David Hildenbrand (14):
  softmmu/physmem: Mark shared anonymous memory RAM_SHARED
  softmmu/physmem: Fix ram_block_discard_range() to handle shared
    anonymous memory
  softmmu/physmem: Fix qemu_ram_remap() to handle shared anonymous
    memory
  util/mmap-alloc: Factor out calculation of the pagesize for the guard
    page
  util/mmap-alloc: Factor out reserving of a memory region to
    mmap_reserve()
  util/mmap-alloc: Factor out activating of memory to mmap_activate()
  softmmu/memory: Pass ram_flags to qemu_ram_alloc_from_fd()
  softmmu/memory: Pass ram_flags to
    memory_region_init_ram_shared_nomigrate()
  util/mmap-alloc: Pass flags instead of separate bools to
    qemu_ram_mmap()
  memory: Introduce RAM_NORESERVE and wire it up in qemu_ram_mmap()
  util/mmap-alloc: Support RAM_NORESERVE via MAP_NORESERVE under Linux
  hostmem: Wire up RAM_NORESERVE via "reserve" property
  qmp: Include "reserve" property of memory backends
  hmp: Print "reserve" property of memory backends with "info memdev"

 backends/hostmem-file.c                       |  11 +-
 backends/hostmem-memfd.c                      |   8 +-
 backends/hostmem-ram.c                        |   7 +-
 backends/hostmem.c                            |  33 +++
 hw/core/machine-hmp-cmds.c                    |   2 +
 hw/core/machine-qmp-cmds.c                    |   1 +
 hw/m68k/next-cube.c                           |   4 +-
 hw/misc/ivshmem.c                             |   5 +-
 include/exec/cpu-common.h                     |   1 +
 include/exec/memory.h                         |  42 ++--
 include/exec/ram_addr.h                       |   9 +-
 include/qemu/mmap-alloc.h                     |  16 +-
 include/qemu/osdep.h                          |  30 ++-
 include/sysemu/hostmem.h                      |   2 +-
 migration/ram.c                               |   3 +-
 qapi/machine.json                             |   6 +
 .../memory-region-housekeeping.cocci          |   8 +-
 softmmu/memory.c                              |  27 ++-
 softmmu/physmem.c                             |  61 +++--
 util/mmap-alloc.c                             | 212 +++++++++++++-----
 util/oslib-posix.c                            |   7 +-
 util/oslib-win32.c                            |  13 +-
 22 files changed, 352 insertions(+), 156 deletions(-)